date: fix parsing months

Thanks nemo for discovering this on #mercurial IRC channel.

Test Plan:
Add a test. It fails before this patch:

```
+  hg: parse error: invalid date: 'Feb 2018'
+  hg: parse error: invalid date: 'Apr 2018'
+  hg: parse error: invalid date: 'Jun 2018'
+  hg: parse error: invalid date: 'Sep 2018'
+  hg: parse error: invalid date: 'Nov 2018'
```

Differential Revision: https://phab.mercurial-scm.org/D2289

mercurial/util.py

@@ -2179,7 +2179,7 b' def matchdate(date):'
             try:
                 d["d"] = days
                 return parsedate(date, extendeddateformats, d)[0]
-            except Abort:
+            except error.ParseError:
                 pass
         d["d"] = "28"
         return parsedate(date, extendeddateformats, d)[0]

tests/test-parse-date.t

@@ -286,3 +286,9 b' Test parsing various ISO8601 forms'
   $ hg debugdate "2016-07-27 121021Z"
   internal: 1469621421 0
   standard: Wed Jul 27 12:10:21 2016 +0000
+
+Test parsing months
+
+  $ for i in Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec; do
+  >   hg log -d "$i 2018" -r null
+  > done